Method for forming robust solder interconnect structures by reducing effects of seed layer underetching

ABSTRACT

A method for forming an interconnect structure for a semiconductor device includes defining a via in a passivation layer so as expose a top metal layer in the semiconductor device. A seed layer is formed over the passivation layer, sidewalls of the via, and the top metal layer. A barrier layer is formed over an exposed portion of the seed layer, the exposed portion defined by a first patterned opening. The semiconductor device is annealed so as to cause atoms from the barrier layer to diffuse into the seed layer thereunderneath, wherein the annealing causes diffused regions of the seed layer to have an altered electrical resistivity and electrode potential with respect to undiffused regions of the seed layer.

BACKGROUND OF INVENTION

The present invention relates generally to semiconductor deviceprocessing and, more particularly, to a method for forming robust solderinterconnect structures by reducing effects of seed layer underetchingwith respect to a barrier layer.

In the manufacture of semiconductor devices, C4 (Controlled-CollapseChip Connection) is a means of connecting integrated circuit chips tosubstrates in electronic packages. In particular, C4 is a flip-chiptechnology in which the interconnections are small solder balls (orbumps) formed on the chip surface. The top layers of an integratedcircuit chip include various wiring levels, separated by insulatinglayers of dielectric material, that provide input/output for the device.In C4 structures, the chip wiring is terminated by a plurality of metalfilms that form the ball-limiting metallurgy (BLM). The BLM defines thesize of the solder bump after reflow, and provides a surface that iswettable by the solder and that reacts with the solder material toprovide good adhesion and acceptable reliability under mechanical andthermal stress. In addition, the BLM also serves as a diffusion barrierbetween the integrated circuit device and the metals in theinterconnection.

As is known in the art, the BLM includes a continuous stack of metalfilms across formed over the wafer to be bumped. This stack of films,also known as a “seed layer” performs a dual function. First, the seedlayer provides a conductive path for current flow during subsequentelectrolytic deposition of the solder bumps. Second, portion of theoriginal seed layer remains under the solder bumps and forms the basisfor the BLM of the C4s. Accordingly, the seed layer must include atleast one layer that is conductive enough to permit uniformelectrodeposition across the entire expanse of the wafer. Moreover, thebottom layer must adhere well to the underlying semiconductor devicepassivation, while the top layer must interact sufficiently with thesolder to form a reliable bond.

While different seed layer metallurgies are available in C4 fabrication,one common combination includes a sputtered titanium-tungsten (TiW)adhesion layer, followed by phased or codeposited chromium-copper (CrCu)and copper (Cu) layers. Once the seed layer is formed over thevia-patterned passivation layer, another photolithographic patterningprocess is implemented for forming the solder material within the maskholes in which the C4 bumps are to be defined. In order to reduce theeffects of dissolution of copper within the solder material after anextended number of reflow operations, a thin nickel/copper (Ni/Cu) layeris also commonly used in the BLM as a barrier therebetween. The Ni/Cubarrier layer is electroplated onto the portions of the seed layer leftexposed following the C4 patterning step.

After the nickel/copper and solder bump materials are plated, thedeveloped resist is removed and the remaining seed layer is removed byetching. More specifically, the Cu and CrCu layers are removed byelectroetching, while the TiW layer is removed by a wet chemical etchprocess. Unfortunately, during the electroetching process, there is anundercut of the Cu and CrCu portions of the seed layer with respect tothe Ni/Cu barrier layer. This results in an incomplete BLM in terms ofthe layers (TiW/CrCu/Cu/Ni) defined under the solder bump. In certaininstances, about 30 to 40% of the outer edge of C4 base is left with aninadequate BLM stack. These defects are problematic in terms of theintegrity of C4 joint reliability.

Accordingly, it would be desirable to implement a process of forming aBLM for C4 solder bump interconnects in a manner that does not result ina substantial underetch of the seed layer with respect to the barrierlayer so as to adversely impact C4 structural integrity.

SUMMARY OF INVENTION

The foregoing discussed drawbacks and deficiencies of the prior art areovercome or alleviated by a method for an interconnect structure for asemiconductor device. In an exemplary embodiment, the method includesdefining a via in a passivation layer so as expose a top metal layer inthe semiconductor device. A seed layer is formed over the passivationlayer, sidewalls of the via, and the top metal layer. A barrier layer isformed over an exposed portion of the seed layer, the exposed portiondefined by a first patterned opening of a first diameter, and a soldermaterial is formed over the barrier layer using a second patternedopening of a second diameter. The second patterned opening is configuredsuch that the second diameter is larger than the first diameter.

In another embodiment, a method for forming an interconnect structurefor a semiconductor device includes defining a via in a passivationlayer so as expose a top metal layer in the semiconductor device. A seedlayer is formed over the passivation layer, sidewalls of the via, andthe top metal layer. A barrier layer is formed over an exposed portionof the seed layer, the exposed portion defined by a first patternedopening. The semiconductor device is annealed so as to cause atoms fromthe barrier layer to diffuse into the seed layer thereunderneath,wherein the annealing causes diffused regions of the seed layer to havean altered electrical resistivity and electrode potential with respectto undiffused regions of the seed layer.

In still another embodiment, a method for introducing a self etch stopmechanism within a metallic thin film includes forming an overlayer uponthe thin film, and annealing the thin film so as to cause atoms from theoverlayer to diffuse into the thin film thereunderneath. The annealingcauses diffused regions of the thin film to have an altered electricalresistivity and electrode potential with respect to undiffused regionsof the thin film.

BRIEF DESCRIPTION OF DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIGS. 1( a) through 1(d) illustrate cross-sectional views of an existingprocess for forming BLM structures for C4 solder interconnects;

FIGS. 2–5 are SEM cross sections of an exemplary C4 bump and BLMstructure formed in the depicted in FIGS. 1( a) through 1(d);

FIGS. 6( a) through 6(e) illustrate cross-sectional views of a methodfor forming robust solder interconnect structures by reducing effects ofseed layer underetching with respect to a barrier layer, in accordancewith an embodiment of the invention;

FIGS. 7( a) through 7(c) are SEM cross sections of a finished, reflowedC4 configured in accordance with the method illustrated in FIGS. 6( a)through 6(d); and

FIGS. 8( a) through 8(f) and FIGS. 9( a) through 9(c) illustrate anotherseries of cross sectional views of an exemplary interconnect formationprocess using an annealing step, in accordance with a further embodimentof the invention.

DETAILED DESCRIPTION

Disclosed herein is a method for forming robust solder interconnectstructures by reducing effects of seed layer underetching with respectto a barrier layer. In one embodiment, an additional photolithographypatterning step is used to separately define the areas in which thesolder bump material is deposited, with respect to the patterning stepused to define the areas for plating the barrier layer. The secondpatterning preferably results in a C4 pattern that is wider than thebarrier layer pattern. In this manner, the subsequent etch of the seedlayer results in an undercutting of the wider solder portions, and notthe barrier layer itself. The wettablilty of the copper seed surfacerelative to the solder material results in a retraction of the solder toa truncated spherical shape upon reflow, with a complete BLM stackthereunderneath.

In another embodiment, the electrical resistivity and electrodepotential of the Cu portion of the seed layer is altered by an annealingstep wherein the Ni atoms from the barrier layer are diffused into thecopper. This in turn modifies the electroetch characteristics of theseed layer, effectively introducing an etch stop into the seed layer. Asis discussed hereinafter, the annealing embodiment may be implemented inconjunction with a pair of photoresist exposure steps using existingphotoresist materials, or may be implemented in a single lithographyexposure step using photoresist material(s) that can withstand anannealing step. In addition, the diffusion annealing approach may alsobe used in combination with the separate, “wider” C4 patterningapproach.

Referring initially to FIGS. 1( a) through 1(d), there is shown a seriesof cross-sectional views of an existing process for forming BLMstructures 100 for C4 solder interconnects. In FIG. 1( a), The structure100 features a metal bond pad 102 (e.g., copper, aluminum) formed at anuppermost metallization level of a semiconductor substrate 104, andwhich serves to provide an external connection to the active devices(not shown) formed in the substrate 104 through various levels ofinterconnect structures therebetween. A final passivation layer 106includes alternating layers of dielectric material (e.g., SiO₂ or low-kmaterial) and pad nitride material (e.g., Si₃N₄), followed by aphotosensitive polyimide (PSPI) layer. For purposes of illustration, themultilayer passivation layer 106 is depicted as a single layer in theFigures.

In order to provide access for a suitable external connection a via oropening 108 is formed through the passivation layer 106 to the topsurface of the metal pad 102. As discussed previously, a seed layer 110(including layers of TiW/CrCu/Cu) is deposited (e.g., by sputtering)over the entire structure, including the sidewalls of the via 108 andthe top of the metal pad 102. As shown in FIG. 1( b), a patterned anddeveloped layer of photoresist material 112 is used to define thelocations in which the C4 connections are to be formed. In FIG. 1( c),both a barrier layer (e.g., nickel, nickel/copper) 114 and the soldermaterial 116 are deposited using the same pattern of photoresistmaterial 112. The solder material 116 may be, for example, a Pb richPb—Sn solder, a Pb—Sn eutectic, or a Pb-free (e.g., Sn rich) solder.

Next the photoresist 112 is removed and the seed layer 110 is etchedback, with the CrCu/Cu portions of the seed layer 110 being removed byelectroetching and the TiW portion being removed by wet chemicaletching. As a result of the electroetching, there is an undercut of theCrCu/Cu portions of the seed layer 110 with respect to the barrier layer114. Therefore, the overall size of the resulting BLM is smaller thanthe base of the solder material, as shown in FIG. 1( d).

FIGS. 2–5 are SEM photographs of a C4 bump and BLM structure, formed inthe conventional manner described above, with an undercutting of theseed layer 110. FIG. 2 shows the C4 structure as a whole, while FIGS. 3,4 and 5 respectively illustrate the middle, left and right portions ofthe C4 and BLM in greater detail.

Referring now to FIGS. 6( a) through 6(e) there is shown, in accordancewith an embodiment of the invention, a method for forming robust solderinterconnect structures by reducing effects of seed layer underetchingwith respect to a barrier layer. In particular, FIG. 6( a) illustrates apoint in the device processing just prior to the solder materialdeposition as was depicted in FIG. 1( c). That is, FIG. 6( a) shows afirst photoresist patterning step used to form a barrier layer 114 overthe blanket seed layer 110 so as to ultimately define the dimensions ofthe BLM and C4 solder ball structures. However, in contrast to theconventional processes, the photoresist layer 112 is stripped prior tothe deposition of the solder material.

Then, as shown in FIG. 6( b), a second photoresist patterning step isused to define a separate C4 pattern, wherein the opening defined by asecond patterned resist layer 118 is larger than that used to define thebarrier layer pattern (i.e., the BLM pattern). Accordingly, when thesolder material 116 is then deposited into the pattern created inphotoresist layer 118, the solder material 116 extends over the outeredges of the barrier layer 116 and directly onto a portion of the seedlayer 110, as shown in FIG. 6( c). In an exemplary embodiment, theopening defined by the second patterned resist layer 118 may be about 20to about 25 microns larger than the opening defined by the firstpatterned resist layer 112.

In FIG. 6( d), the second resist layer 118 is removed and the exposedportion of the seed layer 110 are etched to complete the BLM formation.However, in this instance, the underetching of the seed layer 110 iswith respect to the wider edges of the solder material 116, instead ofthe barrier layer 114. Accordingly, during a reflow process, the soldermaterial 116 retracts in a self-defined manner to the copper layer ofthe seed layer 116 as a result of the wettability property of a coppersurface relative to (molten) solder material. As shown in FIG. 6( c),the reflow process thus insures a truncated, spherical-shaped solderball with a structurally complete BLM TiW/CrCu/Cu stack corresponding tothe dimensions of the barrier layer. Upon reflow, any thin copper atopthe nickel/copper barrier layer, and pail of the nickel is consumed inthe intermetallic formation process. FIGS. 7( a) through 7(c) are SEMcross sections of a finished, reflowed C4 configured in accordance withthe method illustrated in FIGS. 6( a) through 6(d).

In another embodiment of the present invention, the underetch process isaddressed by implementing an annealing approach so as to selectivelyalter the electrical resistivity and electrode potential of the Cu layerunderneath the Ni barrier layer. Since a diffusion anneal (at optimizedtemperature and time conditions) results in Ni and Cu interdiffusion atthe interface thereof, the diffusion of Ni into the Cu locally altersits resistivity and electrode potential. The extent of the Ni diffusioninto the Cu (and thus the change in resistivity) may be controlled byadjusting the duration at which an annealing temperature is applied, inaccordance with the laws of diffusion. Once resistivity and electrodepotential of the Cu layer underneath the Ni barrier layer is alteredwith respect to the surrounding blanket Cu layer, the electroetch rateis slowed so as to create a self etch stop mechanism.

Ni—Cu interdiffusion has been studied for some time. The diffusioncoefficient of Ni tracer in Cu

Ni⁶³ Tracer Diffusion in Cu Data Between 698° C. and 1061° C.

-   -   Activation Energy Q=50.5 Kcal/mol    -   Frequency Factor D₀=0.78 cm²/sec

The diffusion in thin films has been shown to be faster than that inbulk specimens, and thus the above numbers should only be used asgeneral guidelines. However, order of magnitude calculations indicatethat it is possible to achieve substantial diffusion of Ni to a distanceof about one micron (a typical thickness of the blanket thin film in theBLM) at temperatures close to about 350–380° C., at times about 30 to 45minutes. Moreover, the spatial extent of the diffusion is limited to amicron or so in the x, y and z directions. In particular, the electricalresistivity of Cu as a function of atomic % Ni is given in the tablebelow:

Electrical Resistivity of Cu vs. Ni Content (Atomic %) at 300 K

Atomic % of Ni Resistivity (μΩ · cm) Pure Cu 1.5–1.7 Cu + 1.2 Ni 3.0–3.5Cu + 2.16 Ni 4.0–4.2 Cu + 3.32 Ni 5.8 Cu + 10.00 Ni ~15.00 Cu + 25.00 Ni~32.00

As can be seen, there is a dramatic increase in the resistivity of Cuwith Ni content in solid solution. At a Ni content of 10% atomic, theincrease is roughly an order of magnitude.

Therefore, by selecting diffusion conditions such that the extent of Nidiffusion is about one micron in each direction in each direction, theresistivity of the Cu directly underneath the Ni barrier layer (andabout a micron or two around the layer) should be increased by about anorder of magnitude. This in turn defines a volume of Cu that will beelectroetched at a much slower rate compared to the remaining portionsof Cu without any diffused Ni. As will be appreciated by those skilledin the art, electroetching is a well-known metal etching process forback end of line (BEOL) wafer processing. An anodic voltage is appliedto the wafer surface based on the electrolyte composition and the metalto be etched. The metal is preferably etched as anodic current flowsthrough the wafer surface. With regard to electroetching of a BLM seedlayer, the Cu and CrCu portions of the seed layers are removedsimultaneously, while the Ni and solder metals are unaffected. Sincediffused nickel into the copper seed layer would increase the electricalresistivity and passivity of the under bump seed layer, the degree ofcopper undercut is substantially reduced.

Referring now to FIGS. 8( a) through 8(f), there is shown another seriesof cross sectional views illustrating an exemplary interconnectformation process using an annealing step, in accordance with a furtherembodiment of the invention. In FIG. 8( a), a first pattern photoresistlayer 112 is used to form the barrier layer 114 over the blanket seedlayer 110 so as to ultimately define the dimensions of the BLM and C4solder ball structures. In the case where the specific resist materialused is incompatible with a thermal anneal, the first resist layer 112is stripped prior to the diffusion anneal. The diffusion anneal isrepresented by the arrows shown in FIG. 8( b).

On the other hand, if the first lithography step is implemented inconjunction with a lithography coating that can withstand annealingtemperatures (e.g., photoresist polymers, or non-polymeric material suchas silicon oxide TEOS, etc.) and can be patterned by any technique (suchas photo x-ray, laser, lithography and micromachining, for example),then the diffusion anneal could alternatively be carried out withoutfirst removing the developed resist. In that case, additional thin filmdeposition/electroplating steps may be carried out without the use of asecond lithography step, thus leading to cost savings and improved yieldlosses from any misalignment in the second lithography step.

However, in the embodiment where the resist material is sensitive, FIG.8( c) illustrates the patterning of the second photoresist layer 118.The second photoresist layer may be patterned with the same dimensionsas the first photoresist layer 112 (as is shown), or may be intentionalformed wider as described in the earlier embodiment. In FIG. 8( d), thesolder material is deposited. At this point, the second resist layer isremoved, as shown in FIG. 8( e), so that the remaining exposed portionsof the seed layer 110 may be etched for final BLM definition.

As a result of the altered resistivity and electrode potential of thecopper layer portion of the seed layer 110 beneath the nickel/copperbarrier layer 114, there is a created etch stop mechanism as describedearlier. Thus, after electroetching of the Cu and CrCu portions of theseed layer 110, there is no underetching of the seed layer 110 withrespect to the barrier layer 114, and the resulting BLM structure hasimproved structural integrity, as shown in FIG. 8( f).

As also indicated earlier, the elimination of the undercut due to the Nidiffusion profile in the seed layer has an additional advantage in thatthe C4 solder ball will still align to the Ni barrier layer afterreflow, even if a misalignment occurred during a second lithographypatterning step. This additional advantage is illustrated in FIGS. 9( a)through 9(c). In FIG. 9( a), the second photoresist layer 118 is shownslightly misaligned such that a portion of the barrier layer 114 iscovered on a first side 120, and a gap between the resist layer 118 andthe barrier layer 114 is left on a second side 122. When the soldermaterial 116 is deposited, some of the solder material 116 will be indirect contact with the exposed portion of the seed layer 110 on thesecond side 122.

After the second photoresist layer 118 is removed, as shown in FIG. 9(b), the exposed seed layer 110 is etched. Due to the earlier diffusionanneal, there is no undercutting of the copper in the seed layer 110.Thus, the layers of the BLM are generally in alignment at the first side120, while there is an additional section of seed layer 110 underneaththe barrier layer 114 at the second side 122, due to the misaligneddeposited solder material 116. However, after a reflow of the soldermaterial 116, the resulting C4 ball is self aligned to the nickel/copperbarrier layer 114, as shown in FIG. 9( c). Moreover, the BLM isstructurally complete, in that there are no portions of the seed layer110 that undercut the dimensions of the barrier layer 114.

Although the diffusion anneal process is described with reference to C4interconnect formation, it will be appreciated that its application isnot limited to such, and can also be applied to other back end and frontend metallization processes in which tighter ground rules applicationare of concern. For example, any interconnection wherein a soldermaterial is placed on top of a barrier layer such as nickel/copper willresult in self alignment to the barrier layer using this technique,thereby eliminating the need for a second, accurate photolithographyexposure process.

As will also be appreciated, the above described technique of alteringthe resistivity of a layer through diffusion may also be adapted tocreate other general combinations of thin films, wherein alithographically defined overlayer is annealed to result in a structurethat features a self etch stop for etching processes, including anelectroetching process.

While the invention has been described with reference to a preferredembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

1. A method for forming an interconnect structure for a semiconductordevice, the method comprising: defining a via in passivation layer so asexpose a top metal layer in the semiconductor device; forming a seedlayer over said passivation layer, sidewalls of said via, and said topmetal layer; forming a barrier layer over an exposed portion of saidseed layer, said exposed portion defined by, a first patterned opening;annealing the semiconductor device so as to cause atoms from saidbarrier layer to diffuse into said seed layer thereunderneath; andfollowing said annealing, forming a solder material over said barrierlayer using a second patterned opening; wherein said annealing causesdiffused regions of said seed layer to have an altered electricalresistivity and electrode potential with respect to undiffused regionsof said seed layer, and wherein said second patterned opening isconfigured so as to have a lager diameter than said first patternedopening.
 2. The method of claim 1, further comprising etching said seedlayer, following said annealing, so as to define a ball limitingmetallurgy (BLM), wherein remaining portions of said seed layer do notundercut said barrier layer.
 3. The method of claim 1, furthercomprising: following said annealing, forming a solder material oversaid barrier layer using said first patterned opening.
 4. The method ofclaim 3, wherein said first patterned opening is formed using aphotoresist material that is capable of withstanding temperaturesgenerated during said annealing.
 5. A method for forming an interconnectstructure for a semiconductor device the method comprising: defining avia in a passivation layer so as expose a top metal layer in thesemiconductor device; forming a seed layer over said passivation layersidewalls of said via, and said top metal layer; forming a barrier layerover an exposed portion of said seed layer, said exposed portion definedby a first patterned opening, and annealing the semiconductor device soas to cause atoms from said barrier layer to diffuse into said seedlayer thereunderneath; wherein said annealing causes diffused regions ofsaid seed layer to have an altered electrical resistivity and electrodepotential with respect to undiffused regions of said seed layer, andwherein said annealing is implemented at a temperature and a duration soas to cause atoms from said barrier layer to diffuse into said seedlayer by about one micron in x, y and z-directions.
 6. The method ofclaim 5, wherein said annealing results in an increased electricalresistivity of said diffused regions of said seed layer by about oneorder of magnitude.
 7. The method of claim 5, wherein: said seed layercomprises a titanium-tungsten/chrome-copper/copper (TiW/CrCu/Cu) layer;and said barrier layer comprises a nickel/copper layer.
 8. The method ofclaim 7, wherein copper and chrome copper portions of seed layer areremoved by electroetching.
 9. The method of claim 7, wherein saidannealing is implemented at temperature of about 350 to about 380° C.for a duration of about 30 to about 45 minutes.
 10. A method forintroducing a self etch stop mechanism within a metallic thin film, themethod comprising: forming overlayer upon the thin film; annealing thethin film so as to cause atoms from said overlayer to diffuse into thethin film thereunderneath; wherein said annealing causes diffusedregions of the thin film to have an altered electrical resistivity andelectrode potential with respect to undiffused regions of the thin filmand wherein said annealing is implemented at a temperature and aduration so as to cause atoms from said overlayer to diffuse into thethin film by about one micron in x, y and z-directions.
 11. The methodof claim 10, wherein said annealing results in an altered electricalresistivity and electrode potential of said diffused regions of the thinfilm by about one order of magnitude.
 12. The method of claim 10,wherein: the thin film includes a copper layer; and said over-layerincludes a nickel/copper layer.